Database Reference
In-Depth Information
Do you always have to have a phone number?
Can you have more than one phone number?
Do you ever recognize the area code as separate from the rest of the phone num-
ber?
Do you ever need to see phone numbers outside a given country?
What type of phone number is this? That is, is it a fax number, mobile number,
etc.?
Does the time of day matter? For example, do we need to distinguish between the
phone number to use during working hours and outside working hours? Of course,
that would lead to a discussion on what we mean by “working hours.”
To ensure that every attribute is single-valued and provides a fact completely and only
about its primary key, we apply a series of rules in small steps, where each step (or level of
normalization) checks something that moves us towards our goal. Most data professionals
would agree that the full set of normalization levels is the following:
first normal form (1NF)
second normal form (2NF)
third normal form (3NF)
Boyce/Codd normal form (BCNF)
fourth normal form (4NF)
fifth normal form (5NF)
1NF is the lowest level and 5NF the highest. Each level of normalization includes the
lower levels of rules that precede it. If a model is in 5NF, it is also in 4NF, BCNF, and
so on. Even though there are higher levels of normalization than 3NF, many interpret the
term normalized to mean 3NF. This is because the higher levels of normalization (that is,
BCNF, 4NF, and 5NF) cover specific situations that occur much less frequently than the
first three levels. Therefore, to keep things simple, let's focus on only first through third
normal forms.
Initial Chaos
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